This invention pertains generally to the fields of plasma processing and semiconductor manufacturing, and to plasma etching and deposition techniques.
Plasmas are routinely used in the manufacturing of integrated circuits and inicroelectromechanical systems (MEMS). Such plasmas are used for etching of the semiconductor substrates and for the etching or depositing of thin films of materials on the substrates, e.g., films of polycrystalline silicon, silicon dioxide, silicon nitride and metals. The reactive plasmas may be excited in a gas in various ways, commonly by applying a voltage across two electrodes to establish an electric field between the electrodes in a gas at a low pressure. The spacing between the electrodes is typically a few centimeters. The gas is maintained at a pressure low enough such that a plasma is established at a voltage between the electrodes which is below that at which arcing between the electrodes will take place. One of the electrodes may comprise the workpiece on which etching or deposition will take place, while the other electrode may be the wall of the reactor. Typical operating pressures in the plasma chamber are in the range of 1-1000 millitorr, relatively low pressure levels that are necessary to avoid arcing during ignition of the plasma. The requirement for relatively low pressures necessitates the use of fairly expensive vacuum pumps, can require the use of load locks, and can limit the production speed because of the time required to pump down the plasma confinement chamber to the required pressure level.
Silicon etching in commercial plasma processing systems is commonly performed in parallel plate reactors by applying RF power (typically at 13.56 MHz) between two electrodes placed several centimeters apart. The silicon wafer is located on the powered electrode for reactive ion etching. The operating pressure and power are in the range of 10-500 mtorr and 10-500 mW/cm2, respectively. Since the plasma exists globally across the wafer, the etch is selectively masked by a thin film of, e.g., photoresist, SiO2 or metal, which is patterned on the wafer surface. More recently, fast anisotropic etches have been demonstrated by alternative plasma etchers utilizing electron cyclotron resonance (ECR) and inductively coupled plasmas (ICP). All of these options, however, employ a single plasma that acts over the entire surface area of a wafer. Creating several different etch depths or profiles in a single die mandates the use of a like number of masking steps.
A particular challenge in the use of reactive plasmas in semiconductor processing is the need to maintain spatial uniformity in etching or deposition over the entire surface of the semiconductor wafer. Commercial semiconductor wafers have diameters presently as large as 12 inches, with a trend toward increasingly larger wafers. To process such wafers, progressively larger and more expensive reactive plasma systems will be required with the use of conventional plasma processing technology.
In accordance with the present invention, plasma treating to remove material from a surface (e.g., etching) or to add material (e.g., deposition or implantation) or both can be performed over large areas of substrates, such as semiconductor wafers, utilizing spatially localized micro-plasmas operating in parallel with one another. A plasma can be developed in each spatially localized region which is tailored to the plasma treatment requirements of that region, avoiding the non-uniformity of plasma treatment encountered with conventional large area plasma deposition and etching systems, while permitting specific regions of the substrate to receive selected levels of plasma treatment independently of other regions of the substrate. The invention may thus be utilized, for example, to plasma etch some regions of the substrate for longer times than other regions with resulting deeper etches in certain regions than in others, or to provide etches of particular dimensions or patterns. In plasma etching, the power density can be approximately 100 times higher than in conventional plasmas. In addition, DC power can be used to etch the substrates, eliminating the need for matching impedance networks associated with RF driven plasmas. Plasma confinement can be varied from a few tens of microns to more than a centimeter by changing operating conditions. The electrodes for the micro-plasmas may also serve to mask the etch in regions where the micro-plasma is ignited. The etch dimensions are consequently confined to the openings in the mask, allowing as precise masking of the etched areas as in conventional etching. For deposition processes, the invention may be utilized to allow plasma mediated deposit of different materials in various regions of the substrate in a pattern. For example, a plasma may be established at certain of the spatially separated regions of the substrate while a first precursor gas is supplied to the region, and then a plasma may be established in other regions of the substrate while a second precursor gas is supplied, allowing multiple plasma deposition processes to take place without requiring separate lithography masks or removal or replacement of masks.
In the present invention, a plasma generating electrode is positioned closely adjacent to an exposed surface of the substrate, such as above the surface or laterally spaced from the surface. A selected pressure of the gas in the region of the electrode and the substrate is established, and a voltage is applied between the electrode and the substrate to ignite a plasma in the region between the electrode and substrate for a selected period of time. The plasma is limited to the region of the electrode adjacent to the exposed surface so that the substrate is plasma treated in a pattern defined by the electrode. The electrode may be formed as separated electrode segments which are held over and spaced from the surface of the substrate so that a plasma may be established between the electrode and the substrate in the ambient gas surrounding the electrode and substrate. An electrode patterned in this manner may be selectively moved around the substrate, either continuously or stepwise, to provide patterned etching or deposition treatment of the substrate surface. A single electrode may also be used as a probe to plasma treat the substrate as the probe is moved over the surface of the substrate. The various segments of the electrode may be independently supplied with voltage so that different voltage levels may be applied between the electrodes for different lengths of time to tailor the amount of plasma etching or deposition at particular locations on the substrate. The electrode may also be formed by utilizing a dielectric layer in contact with the surface of the substrate with openings therein, with the electrode formed on the dielectric layer such that a plasma is established in the pattern of 1 openings in the dielectric layer as a voltage is applied between the electrode and the substrate. Separate electrodes which may be separately supplied with voltage may be formed at or adjacent to the various openings in the substrate to allow tailoring of the plasmas at specific regions of the substrate. The dielectric layer and electrode may be formed separately from the substrate and mounted onto the substrate at a particular position at which the plasma treatments are to be performed, removed from a position on that substrate, and then applied to a new substrate, or may be moved in a stepwise fashion from position to position about a single substrate to provide a repeated selected pattern of plasma treatment over the surface of the substrate. The dielectric layer may also be formed as a layer in situ on the substrate, with the electrode formed over it either permanently or subject to subsequent removal. The dielectric layer may be formed directly on the surface of the substrates, or a second base electrode may be formed on the substrate surface and the dielectric layer formed over it so that the plasma generation or control voltage can be applied between the upper electrode and the lower base electrode.
An electrode formed in this manner in permanent position on the substrate may be encapsulated in a casing for the substrate, e.g., a completely processed semiconductor chip, with leads extending from the electrode and the substrate to leads or pins outside the casing. Electrical voltage may then be applied selectively to the external pins at a later time to carry out additional plasma treatment, e.g., selected etching of regions of the substrate to tailor the performance of the completed packaged unit, for example, by etching to trim the resistance of a resistor on the semiconductor substrate.
In the present invention, the plasma generating electrode may be formed as first electrode on top of a dielectric layer, with a second plasma generating electrode also formed on top of the dielectric layer spaced from the first electrode by an opening in the dielectric layer. A relatively high voltage is applied between the two plasma generating electrodes to produce a plasma in the region between them. At least one control electrode may be formed on the exposed substrate surface in the region in which the plasma is generated and be biased separately from the plasma generating electrodes to control the application of the plasma to the substrate. The separate control electrode allow various areas of the substrate to be etched or deposited at different rates or for different lengths of time.
A particular advantage of the present invention is that the spacing between the electrode and the substrate may be, and preferably is, small-preferably 1000 xcexcm or less, and preferably in the range of 0.1 to 1,000 xcexcm-allowing relatively high electric fields to be developed between the electrode and the substrate with relatively low applied voltages. In addition, plasmas can be developed at gas pressures that are much higher than those that are required for conventional plasma processing, typically at least 1 torr, while still avoiding arcing between the electrode and the substrate. The higher operating pressures can also assist in confining the plasma volume. The higher operating pressure enabled by the use of the present invention reduces the need for expensive vacuum pumps and allows shorter processing times.
The present invention allows highly tailored micro-plasmas that are well suited to applications where etch rate, directionality and the like must be controlled for localized areas. The ability to separately control spatially distinct micro-plasmas operating in parallel with each other over a wafer surface allows directionality and other features of the plasma to be controlled in the localized regions. As an example, an isotropic etch may proceed in one region while an anisotropic etch is performed in another region, enhancing manufacturing throughput. Appropriate control circuitry may be utilized to control the voltages applied to electrode segments to separately modify the spatially separated micro-plasmas over time, allowing customization of the etch regions and permitting fabrication of features that might otherwise require hundreds of lithography steps. Such control circuitry may, if desired, be integrated on the semiconductor surface being etched, allowing an in situ self-controlled etch. The invention may also include circuitry for detecting the endpoint of a plasma etch or deposition process in a region and for terminating the etch or deposition when the desired endpoint has been reached.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.